1. Field of the Invention
The present invention relates generally to aerosol sampling methods and devices for sampling particulate matter in ambient air, and more specifically to a flow rate control method and apparatus for use with a filter.
2. Description of the Prior Art
It is now generally recognized and accepted that particulate matter in air, especially particles smaller than 10 micrometers in diameter, which can be inhaled easily and deposited in the tracheo-bronchial regions of the human respiratory tract, pose substantial health concerns. Consequently, the United States Environmental Protection Agency (EPA) has established a National Ambient Air Quality Standard in 52 Fed. Reg. 24634-24735, 40 CFR Part 50, which strictly limits amounts of particulate matter of less than 10 micrometer size in ambient air, since the principal target is the human lung, a long term integrating receptor. This National Ambient Air Quality Standard is expressed in terms of maximum allowable ambient concentrations of particles having aerodynamic diameters smaller than a nominal 10 micrometers (PM.sub.10) for both annual and daily time periods.
Accurate, precise and reliable monitoring equipment is required to demonstrate compliance or noncompliance with the EPA Ambient Air Quality Standard. Such monitoring equipment needs to be able to take in samples of particle-laden ambient air in a consistent, representative manner over the specified sample or test time period, separate the particles smaller than 10 micrometers (PM.sub.10) from the larger particles, and collect the PM.sub.10 particles for measuring, weighing, or other analyses. My U.S. Pat. No. 4,461,183 illustrates an aerosol sampler inlet that meets the EPA specifications for collecting ambient particle samples and separating PM.sub.10 particles from larger particles in the sample for passing on to an appropriate PM.sub.10 particle collector, which has been specified by EPA in 40 CFR Part 50 to be a filter medium having a specific efficiency for test particles and certain physical and chemical properties. It was also determined, however, that the accuracy of such sampler inlets requires specific and constant air velocities throughout the sampling time period, regardless of varying ambient conditions of wind velocities and directions, air temperatures, and quantities of PM.sub.10 particulate matter trapped in or on the filter medium. Consequently, the EPA rule in 40 CFR Part 50, Appendix J, requires that ". . . specific air velocities be maintained in the sampler's air inlet system." 52 Fed. Reg. 24665. Therefore, the volumetric flow rate through the sampler's inlet must be maintained constant, within inlet design tolerances, throughout the sampling period. The flow controller method and apparatus disclosed in my U.S. Pat. No. 4,649,760 utilizes a critical (choked) flow venturi to achieve such constant volumetric flow rate control for accurate aerosol sampling and monitoring, as required by the EPA regulations.
While a 24-hour mean concentration of PM.sub.10 is a useful measure of particle concentration as it affects health, as described above, it is less useful for identifying specific diurnal patterns in mass concentration of the particles or for identifying emission sources of specific particles within the standard 24-hour sampling periods. Consequently, even more sophisticated sampling is required to identify given species or elements of particles captured in the ambient aerosol samples, rather than just the total mass of PM.sub.10 particles deposited onto the sampler's filter medium in a 24-hour period. More sophisticated aerosol sampling is also required to identify sources of such air-borne particles, for example, by collecting and recording more sampling periods and perhaps in sampling periods of shorter duration, such as every hour.
Unfortunately, the fibrous glass or quartz filter media customarily used in monitoring total PM.sub.10 mass collected over a 24-hour period cannot be used effectively for numerous, shorter sample periods where it is desired to identify and quantify given particulate or elemental species collected from the ambient air during each period. Such filters are quite friable an tend to lose portions of the edge fibers during handling, requiring a large filter to minimize the impact of these losses. This in turn requires a long sampling time to collect enough particulate matter. In addition, such filters are rather soluble, and thus interfere with many analytical measurements, and many of them also contain variable amounts of the elements it is desired to measure. Their structure also does not permit microscopy of the collected samples.
Numerous other filter types are available, including membrane filters (non-fibrous filters with variously formed pores of typical mean diameters from about 0.1 to about 10 micrometers), fabricated from a variety of polymeric materials from cellulose esters to PTFE ("Teflon" trademark), and including PVC (polyvinyl chloride) and polycarbonate ("Lexan" trademark). Each of these filter materials is a filter of choice for some purposes in the measurement of atmospheric pollution. Each of them has different characteristics of flow rate versus pressure drop, and all of them have markedly higher pressure drops at any given flow rate than fibrous glass or quartz filter media. These pressure drops for such membrane filter media are sufficiently high that neither the flow controller nor the air mover of my method and apparatus for controlling flow volume through an aerosol sampler described in my U.S. Pat. No. 4,649,760 can function as required using any practical filter size. Also, as shown by the U.S. Pat. No. 4,961,916, issued to J. Lesage, et al., there are also specific types of filters with special chemical impregnation for sampling toxic vapors, which also have different and varying flow rate to pressure drop characteristics.
At the present time, at least prior to this invention, there were no samplers available that could collect PM.sub.10 particulate matter or other airborne liquid or solid matter using the same sampler on a variety of filter substrates while maintaining constant volumetric flow rate of the air flowing through the sampler for the duration of the desired or needed sampling periods, as required by 40 CFR Part 50, Appendix J, or as required by the sampler inlet design specifications for maintaining accurate PM.sub.10 particle separation or other criteria..